Skip to main content

Full text of "The Cambridge Natural History"

See other formats



the former is alone concerned with the glenoid cavity. It must
therefore, one would suppose, correspond to the " coracoid " of the
Monotremata, while the upper piece of bone is the epicoracoid
process of that mammal. The Mammalia, therefore, higher as
well as lower, differ from the reptiles in that the coracoid is
formed of two bones, the exceptions being, among some other
extinct forms, certain of the Anomodontia, a group which it will
be recollected is the nearest of all reptiles to the mammals.

The Fore-limb.—The  humerus  is  of varying  length among
mammals.     A feature which it sometimes shares with the humerus

FIG. 30.—Distal extrem-
ity of tlie hnmerus to
show E] >icondylar Fora-
mina. A, In Hatteria ;
B, in a Lizard {Lacerta
ocettata) ; C, in the
Domestic Cat ; "D, in
Man. c.e, External con-
dyle ; c.iy internal con-
dyle. In A the two
foramina are developed
(at t, the entepicondy-
lar ; at ii, the ectepi-
condylar). The only
canal (f) present in the
Lizard (B) is on the
external ulnar side, in
the cartilaginous distal
extremity. In Man (B)
an entepicondylar pro-
cess (pr) is sometimes
developed and con-
tinued as a fibrous band.
(From Wiedersheim's
Anatomy of Man.)

of lower forms is the presence of an entepicondylar foramen, a
defect of ossification situated above the inner condyle of that bone
which transmits a nerve. The same foramen and an additional
ectepicondylar foramen are found in the ancient reptilian type
JTatteria (Sphenodori) ; it occurs also in the Anomodont reptiles.
It is as a rule only the lower forms among mammals which show
this foramen; thus it is present in the Mole and absent in the